1. Field of the Invention
The invention relates to protective coatings for blade and, more particularly, to a method for coating a hollow, internally cooled blade of a gas turbine and a blade of this type for a gas turbine.
2. Description of the Related Art
In modern gas turbines, almost all surfaces in the hot-gas section are provided with coatings to protect them from high-temperature oxidation and high-temperature corrosion. An exception to the foregoing, however, is the case in which the turbine blades are located in the rear of a blade array. Chromium oxide-forming and aluminum oxide-forming diffusion layers and cladding layers have become widely adopted for such applications. In the latter case, MCrAlY cladding layers (M=Ni, Co) are the type of cladding most widely used in stationary gas turbines.
Simultaneously, cooled blades (i.e., hollow blades) are being produced to an increasing extent to keep the material temperature to a level that the available materials can withstand. Cooling is usually accomplished with compressed air, which, depending on the pressure in the environment of the blades to be cooled, is obtained from certain stages of the compressor of the gas turbine system. The temperature of the cooling air is between approximately 450° C. at the inlet to the blade and approximately 800° C. at the outlet from the blade. In the case of highly stressed cooled blades, internal coatings are being used to an increasing extent to prevent oxidation-caused attacks on the grain boundaries, which can have the effect of initiating cracks. Through the use of internal coatings in blades, the thermomechanical fatigue (TMF) life can be considerably extended.
For reasons of process technology, Al diffusion layers produced by alitization have therefore been used almost exclusively. The disadvantage of an Al diffusion layer as an internal coating is that the Al diffusion layer is brittle and has a relatively low resistance to the sources of high-temperature corrosion, which can be present as contaminants in the cooling air. It is also known that Cr diffusion layers produced by chromizing can be used to protect against high-temperature oxidation and corrosion.
In addition, heat insulating layer systems are used on cooled components, such as the blades of gas turbines. Heat-insulating layer systems in gas turbines always consist of a metallic bonding layer diffusion bonded to the base material, on top of which a ceramic layer with poor thermal conductivity is applied, which represents the actual barrier against the heat flow.
In principle, either diffusion layers or cladding layers of the MCrAlY type can be used as bonding layers. The most important property of the bonding layers is the ability to form the purest possible aluminum oxide on its surface, as a result of which the cyclical and static bonding of the heat insulating layer system is ensured.
MCrAlY layers contain the intermetallic β-phase NiCoAl as an aluminum reserve in a NiCoCr (“γ”) matrix. The β-phase NiCoAl, however, also has an embrittling effect. As a result, the Al content which can be realized in practice is ≦12 wt. %. To achieve a further increase in the oxidation resistance, it is possible to coat the MCrAlY layers with an Al diffusion layer by alitization. Because of the danger of embrittlement, this is limited in most cases to starting layers with a relatively low aluminum content.
To increase the Al content in the MCrAlY layer, it is proposed in German Patent Application 10 2004 045 049.8 to continue the alitizing until an Al diffusion layer with an Al content of approximately 20% and on top of that a built-up Al layer of approximately 30% has been produced (by further alitization). The built-up Al layer contains the very brittle β-NiAl phase. By means of an abrasive treatment, e.g., by blasting with hard particles, the outer built-up Al layer is removed down to the Al diffusion layer, as a result of which the Al content in the remaining diffusion layer is at least 18% and no more than 30%.